Added Polish language to i18n configurations.

[Ryszard-Trojnacki]

Description

Polish translations are 100% ready in Crowdin LiteFarm project and I think they can be included in LiteFarm relase.
I have prepared PR with configuration changes to code to include Polish language to LiteFarm.

This PR only adds pl to language lists and one option for language select.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Passes test case
• UI components visually reviewed on desktop view
• UI components visually reviewed on mobile view
• Other (please explain)

I have installed LiteFarm on our server for 3 weeks and everything works ok with Polish language.

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The precommit and linting ran successfully
• I have added or updated language tags for text that's part of the UI
• I have added "MISSING" for all new language tags to languages I don't speak
• I have added the GNU General Public License to all new files